In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Commercial nuclear fuel can be of many forms specific to a particular reactor type though essentially all commercial power reactors utilize uranium as the initial fissile material. The most common fuel type is the uranium oxide (UO2) pellet housed within a thin zirconium alloy cladding of a light water reactor (LWR.) This fuel type is used in both LWR variants: the pressured water reactor (PWR) and the boiling water reactor (BWR) configuration. This UO2 pellet is mass-produced through a conventional ceramic processing route. Once a powder of appropriate purity and enrichment is achieved it is pressed and then sintered in the presence of hydrogen and taken to final dimension by center-less grinding. A very similar process to arrive at UO2 in zircaloy clad is followed for the production of the CANDU (Canada deuterium-uranium) heavy water moderated reactor fuel, though the starting powder can include natural enrichment, recycled uranium (RU), or mixed oxide (MOX). The CANDU and LWR's make up the vast majority of the present international nuclear power fleet making UO2 in zircaloy clad the dominant nuclear fuel system. Arguably, the zircaloy clad of this fuel is the primary fission gas barrier in these LWR and CANDU systems.
The high-temperature Gas-Cooled Reactors (HTGR's), yet to become a significant commercial nuclear platform, whether in the prismatic or pebble-bed configuration, utilize a fuel specifically engineered as a primary barrier to fission product retention. This is achieved through engineering layers of carbon, graphite and SiC around the UO2 (or other) fuel kernel such that the SiC becomes a pressure vessel. This structure, otherwise known as a TRISO (Tri-Structure Isotropic) fuel is combined with many such small spheres on the order of ˜1 mm in diameter which are then compacted (pressed) into a host graphite matrix and has been used in a small number of commercial power reactors. A primary safety advantage of such a fuel is the elimination of the zircaloy cladding which can interact with the coolant under certain accident conditions.
More recently, a fuel form has been developed whereby TRISO, rather than being compacted in graphite as is the case for HTGR, is compacted within a strong and impermeable silicon carbide (SiC) matrix. This relatively new TRISO-based, SiC matrix fuel is referred to as fully ceramic microencapsulated (FCM) fuel. Intermixing a plurality of TRISO particles within such a SiC matrix results in two barriers to fission product release, significantly enhancing the safety aspects of nuclear fuel as compared with the LWR-standard UO2-zircaloy or the HTGR-standard TRISO-graphite compact.
While the HTGR TRISO and FCM arguably convey safety benefits to nuclear systems both the TRISO itself and the methodology by which the FCM is processed are more complex as compared to the UO2 process resulting in fuel which is either marginally more expensive, or for the case of FCM, potentially impractical for mass production. Presently, due to the relatively high temperature and pressure required for the processing of FCM, hot-pressing was the preferred method, which is not a process which lends itself to the mass-production levels required for nuclear fuel. Moreover, given the heterogeneous nature of the fuel, the centerless grinding step common to UO2 and current FCM process is problematic in that exposure of the TRISO kernel is undesirable. Accordingly, there remains a need for an improved method of forming enhanced fission fuel in the form of fully ceramic microencapsulated TRISO-based fuel.